When you are staking, you are in a race in a lottery system.
When staking, you are actually generating blocks that also need to be accepted by the rest of the network.
If the network accepts your block, you are rewarded with your stake amount.
If the network doesnt accept your block, then your coins for that stake are returned.
As the race to accept the block finally comes down to which client can get theirs in first. If someone gets in before you and theirs gets accepted, your block will become conflicted.
All’s not lost however, the coinage is maintained for those coins, and will be used to calculate your next reward.